Two-photon spectroscopy will be used to identify the location and photophysical properties of the low-lying "forbidden" covalent pi state in various visual chromophores and chromophore analogs, the visual pigment rhodopsin, and the light adapted form of bacteriorhodopsin. Two-photon spectra will be obtained in a variety of solvent environments to help define the dipolar and dispersive characteristics of the "forbidden" states. Two-photon polarization data will be obtained to help define the symmetry characteristics of these states. All-valence electron molecular orbital theory will be developed to help interpret the spectroscopic and photochemical properties of the excited electronic states of the visual chromophores. These procedures will be optimized by including high levels of both single and double excitation configuration interaction. The optimized molecular orbital procedures will be used to calculate one-photon and two-photon properties and to generate potential energy surfaces for cis-trans isomerization of the isolated and protein-bound visual chromophores. Our semiclassical molecular dynamics program will be modified to handle multi-dimensional surface trajectories with the goal of more accurately simulating the primary photochemical event of visual transduction. An important goal of our proposed research is to spectroscopically and theoretically investigate the hypothesis that the primary event in vertebrate visual transduction involves a barrierless cis-trans isomerization (Birge, R. R. and Hubbard, L. M. (1980) J. Am. Chem. Soc., 102, 2195). Our emphasis on the spectroscopic analysis of the "forbidden" covalent pi state is associated with the theoretical prediction that this excited state is responsible for the barrierless excited state potential surface of the 11-cis protonated Schiff base chromophore in rhodopsin. This investigation will afford the first detailed analysis of the location and conformational dependence of the low-lying "forbidden" levels in these compounds, and will help delineate their importance in the vision process.